Meaning
Blending ratios for reprocessed thermoplastic scrap determine the recycled content of the molded part. In most molding operations, the regrind fraction is controlled to balance material cost savings against the risk of part failure. It represents the percentage of runners, sprues, and scrap parts that are granulated and returned to the hopper.
While high ratios reduce raw material costs, they can degrade the mechanical properties of the finished parts. This proportion is typically limited to twenty or thirty percent to maintain part performance.
Blending Ratio
Molders use gravimetric blenders to maintain a precise ratio of regrind during the run. Fluctuations can lead to inconsistent melt viscosity and dimensional variation. Accurate control is essential to prevent process drift.
Material Property
Each cycle through the molding machine reduces the molecular weight of the polymer due to thermal and shear stress. A high fraction of regrind can lead to brittle failures, color shifts, and reduced impact strength. Molders test the tensile and impact properties of the parts to determine the maximum acceptable ratio.
This step prevents field failures.
Process Stabilization
To hold dimensions across a run, the feed composition must remain constant. If the regrind fraction varies, the molding parameters must be adjusted to compensate for viscosity changes. Automated blending systems prevent these variations by continuously adjusting the feed rate of each material.
This maintains a stable process and consistent part quality.
